When DSF levels are low (left), the interaction between RpfC and RpfF directly interferes with the production of additional DSF. As bacterial density in the environment increases (right), elevated DSF levels trigger the release of RpfF and stimulate further DSF synthesis.
© 2010 Elsevier
Many bacteria are remarkably civic-minded, altering their behavior in response to changes in the density of their surrounding population. This collective organization is achieved through ‘quorum-sensing’ systems, which respond to molecules secreted by individual bacterial cells; once a threshold level of signaling activity is reached, bacteria respond by switching on specific quorum-responsive genes.
Several years ago, Lian-Hui Zhang and co-workers at the A*STAR Institute of Molecular and Cell Biology (IMCB) identified a novel quorum-sensing pathway, driven by the diffusible signaling factor (DSF), which appears to be employed by a number of plant and animal pathogens. Subsequent collaboration between Zhang and his IMCB colleague Haiwei Song has now revealed important details about the mechanism that enables DSF to modulate its own production and switch on genes associated with bacterial virulence.
This process is governed by the DSF sensor RpfC, which detects quorum signals and transmits them to the DSF-manufacturing enzyme RpfF. Song, Zhang and their co-workers began by characterizing the structural details of the RpfC–RpfF complex, revealing that RpfF belongs to a family of enzymes known as enoyl-CoA hydratases/dehydratases. They also identified a number of specific amino acids with essential roles in DSF production, most of which are positioned within a narrow pocket in the protein that most likely accommodates DSF precursor molecules.
RpfC contains a signaling domain known as REC, which binds to RpfF and thereby inhibits DSF production, and the researchers also identified key surface elements that mediate this interaction. Intriguingly, REC binding appears to lock RpfF into a configuration that leaves its catalytic pocket inaccessible, thereby physically preventing the production of DSF for as long as REC remains bound. Once DSF accumulation reaches the ‘tipping point’ in the external environment, however, signaling through RpfC triggers chemical modification of the REC domain, causing it to dissociate and leading to a sharp increase in DSF production and quorum signaling (see image). “Control of signal production is usually at the level of transcription,” says Zhang. “The most interesting finding of this research is the structural demonstration of a novel, post-translational mechanism for quorum-sensing signal autoregulation.”
As DSF signaling through RpfC also switches on the production of virulence factors that enable pathogens to get down to their dirty work, this pathway represents a potentially important therapeutic target. “Work is ongoing to identify the substrates and catalytic mechanism of RpfF,” says Zhang. “The identification of key residues associated with the sensor-synthase interaction has already provided useful clues for future drug design.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Molecular and Cell Biology.
